Investigators said three children had climbed Mikkeli's Tuppurala primary school roof, where they had played with fire.
Police said it was likely that the fire was still smoldering when the kids finished playing and climbed down. The school’s gym and roof were destroyed in the blaze that broke out on Wednesday evening.
No one was reported injured in the incident, as school is out of session for the summer.
Officials said the three young suspects had cooperated with police, and that authorities did not need any more tips from the public pertaining to the case.
Police are treating the incident as a case of aggravated vandalism.
The primary school serves about 180 pupils and is located near Saimaa Lake, about two kilometres east of the centre of Mikkeli.